Swallowing
Swallowing is the complex process by which food travels from the oral cavtiy to the stomach. It consists of 3 stages - oral, pharyngeal, and oesophageal. Failure of the process can be caused by a range of things and can result in choking or pulmonary aspiration, which have many dangerous secondary effects. The phases of swallowing Each phase is controlled by different areas of the brain. The oral phase is entirely voluntary and is mainly controlled by the medial temporal lobes and the limbic system with other cortical contributions. The pharyngeal swallow is started by the oral phase and coordinated by the swallowing centre in the medulla oblongata and pons. The reflex is stimulated by a bolus of food/liquid activating touch receptors at the back of the mouth or palate. 'Oral phase' #Moistening - Food is moistened by the saliva from salivary glands (parasympathetic activation) #Mastication - Food is mechanically broken down by chewing (muscles of mastication and teeth). The buccinator prevents food from going between outer surfaces of teeth and the cheeks. A bolus is food ready to be swallowed, and anything not largely held together by saliva (mostly mucus) will not be swallowed. #Trough formation - The intrinsic muscles of the tongue form a trough at the back of the mouth and force the bolus into it. Mylohyoid and the glossus muscles elevate the tongue to facilitate this. #Posterior bolus movement - The bolus has been formed. Orbicularis oris contracts to close the mouth. Superior longitudinal muscle elevates the apex of the tongue, pushing the bolus to the back of the mouth against the hard palate. Proprioceptive receptors in the palatoglossal arch of the oropharynx are activated and start the pharyngeal phase (afferents are CN IX, efferents are CN IX and X). 'Pharyngeal phase' #Nasopharynx closure - tensor and levator palatini work in conjunction to close of the nasopharynx. #Pharynx preparation - The suprahyoid and longitudial pharyngeal muscles (stylo- , salpingo-, and palatopharyngeus), pull the pharynx up and forwards. The palatopharyngeal folds on either side of the throat are pulled closer by the superior constrictor muscles, preventing large boluses from passing. #Auditory tube opening - The actions of levator and tensor palatini, and salpinopharyngeus also cause the Eustachian tube to open, allowing the pressure between the middle ear and nasopharynx to equalise. #Oropharynx closure - The palatoglossus, styloglossus, and intrinsic tongue muscles all help to close off the oropharynx. #Laryngeal closure - true vocal cord closure is the 1' method of protection against pulmonary aspiration during swallowing; lateral cricoarytenoids and transverse arytenoids facilitate the complete adduction. Swallowing occurs during expiration usually (even at full expiration, a fine air jet is expired), to clear the larynx of any remaining undesired matter). Clinical significance here is that patients with already compromised lung function will develop increasing respiratory distress as a meal progresses. Retroversion of the epiglottis then takes place (as a 2' form of protection against aspiration). The respiratory centre of the medulla is directly inhibited by the swallowing centre for the time it takes to swallow (referred to as deglutition apnoea). #Hyoid elevation - The stylohyoid an both parts of the digastric contract to lift the pharynx and larynx up even further. #Bolus transits to pharynx - Peristaltic contractions of the sup., mid., and inf. constrictor muscles move the bolus through the pharynx. 'Oesophageal phase' #Peristalsis - The bolus is propelled downwards through the oesophagus by striated muscle (recurrent laryngeal), then by smooth muscle. The upper oesophageal sphincter relaxes to allow food to pass, and peristaltic contractions and relaxation of the lower oesophageal sphincter then push the food into the stomach. #Relaxation - The larynx, pharynx, and hyoid move back down mostly by elastic recoil. Though swallowing is initiated by a voluntary process, it is near impossible to stop once the involuntary phases are reached.